Cooling unit



April 29, 1952 B. LEMESHKA COOLING UNIT Filed Maon :51, 1949 III IHH llllll I llllllllllwl,

l l l INVENTOR MILTON B. LEMESHKA BY MM A RNEY Patented Apr. 29, 1952 COOLING UNIT Milton B. Lemeshka, Lancaster, Pa., assigner to Radio Corporation of America., a corporation of Delaware Application March 31, 1949, Serial No. 84,631

10 Claims.

This invention relates to cooling units through which a heat exchange medium is passed and more particularly to such units which are subject to temperature cycling in normal use.

For the purpose of illustrating my invention it will be described in connection with an electron discharge device having an external surface or electrode to be cooled, although it is to be understood that it is not limited to that particular construction. In accordance with the common practices embodied in the design of external cooling units either a gas or liquid is utilized as the heat exchange medium in conjunction with a receptacle which is designed to receive the portion to be cooled therein. The problems attending the cooling of such devices have become progressively more diflicult as the power output and frequency has been increased. In the case of a liquid medium which is passed through a conduit such as a coiled pipe or the like, the problem of effecting a good thermal bond between the conduit and the cooled surface is present. In the case of forced air cooling an array of cooling fins is generally connected to a core which may serve as a receptacle for the portion of the device to be cooled. Such a unit is shown in my co-pending patent application Serial Number 68,335 filed December 30, 1948 and in the co-pending application of Anthony G. Nekut, Serial Number 68,173 led December 30, 1948, both assigned to the same assignee as the present application.

Such forced air cooling units have been generally made of copper because of its good thermal characteristics and because of the problem of effecting a good thermal bond between the cooling unit and the cooled device which in practice has an external copper electrode. However, asv the size and hence the weight of such units has'increased the cost thereof as well as the substantial weight has become a serious disadvantage. It has been proposed and attempts have been made to form the cooling unit of a lighter metal such as aluminum. Though such units may be readily fabricated from aluminum, the problem of effecting a bond between the aluminum core, receptacle, or conduit and the copper electrode of the electron discharge device which would survive temperature cycling has up to now proven unsurmountable. At 100 C. the coefficient of thermal expansion of aluminum is 235 6 while for copper it is 166 l06. Thus, though a cooler of conventional design may be readily fabricated from aluminum and soldered to a copper electrode, once the solder has solidified and remains solid the temperature of the electrode and cooler cannot be varied without setting up mechanical stresses in the region of the solder joint, because of the difference in expansion between the aluminum and the copper. The magnitude of the stresses developed in the normal operating temperature cycle exceeds the mechanical strength of all known soft (melting point 300-250 C.) solders. Deterioration of the thermal bond between the cooler core and the electrode manifests itself by an increasing temperature drop between the two and a rise in electrode temperature beyond permissible limits.

Thus, a principle object of my invention is to provide a cooler considerably reduced in cost and of improved efficiency.

Another object is to provide a mechanical bond between the cooler and the device or part thereof to be cooled which effectively connects the two under static temperature conditions normally encountered and yet which provides a high performance thermal bond during operation of the cooler and the device which will overcome any tendency of stress to develop between the cooler and the device.

A still further object is to provide an electron discharge device with a liquid or forced air cooler having a thermal coeicient of expansion substantially different from the device which is normally mechanically connected thereto and which when the device is put into operation has a good thermal bond therewith in spite of any amount of temperature cycling.

Yet another Object is the provision of an improved thermal bond between a conduit for a liquid coolant and the part of an electron discharge device'to be cooled.

Stillother objects will appear as the nature of my invention is more fully understood from the following description taken in conjunction with the accompanying drawings wherein like parts are designated by identical numerals in the several views.

In carrying out my invention, 1 utilize a fusible alloy to join the cooler to the part of the device to be cooledsuch as the anode of an electron discharge device, the alloy havingsuch a melting point that it is liquid at the operating temperature of the cooler and electron discharge device. A heating element may be utilized to provide means for heating the fusible alloy, particularly where relatively large quantities of the alloy are used, to facilitate removal of the device from the cooler when the device is not in operation.

.The novelfeatures whichl believe to be characteristic-ofmy.inventiomare set forth with particularity in the appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawing in which:

Figure 1 is a longitudinal sectional View, partially in elevation of an electron discharge device and a forced air cooler therefore constructed in accordance with my invention and showing only those parts necessary to an understanding of my invention;

Figure 2 is a view similar to Figure l of an electron discharge device and a liquid type cooler; and

Figure 3 is a plan View of the cover for the central portion of the cooler shown in Figure l.

In view of the weight or cost of a cooler for an electron discharge device, the use of a lighter metal such as aluminum for the fabrication of the cooler rather than copper as was the usual practice, has great advantages since the Weight of aluminum is approximately 30% that of copper while the thermal conductivity thereof is substantially 55% of copper. It is evident that an aluminum cooler will weight only approximately 54% as much as its equivalent made of copper and with the cost of these metals being substantially the same at the present time an equivalent saving in cost is also made.

For the purpose of illustrating my invention, I have shown part of an electron discharge device I having, as a part thereof to be cooled, an external copper anode I I with an annular ange I2. The cooler which is fabricated entirely of aluminum has a receptacle-like portion or core I3 to which may be assembled ns I4. The particular construction and arrangement of fins M about receptacle-like portion or core I3 forms no part of my present invention and may be carried out in any desired manner. As il1ustrated in Figure l, such a forced air type cooler may be constructed as is fully disclosed in the co-pending applications referred to hereinabove, Lemeshka, Serial No. 68,335 or Nekut, Serial No. 68,173, with a plurality of hns I4, arranged in a radial array, having complementary flanges E5 which nest with the ilanges of adjacent fins. Flanges I5 are joined to one another and to the core by means of soldering, brazing or the like.

My invention is not limited to forced air coolers as the heat exchange means but is also applicable to liquid coolers utilizing a liquid heat exchange medium such as water as will presently appear. In carrying out my invention with respect to such forced air coolers, I provide a ree ceptacle-like core I3 which is closed at one end and open at the other. The open end of the core and a portion I3 thereof for some distance toward the closed end has an increased diameter formed by a transverse shoulder I6 and which acts as a well. The outside diameter of the electrode II is sumciently less than the inside diameter of the core as to permit free iiow of solder Il' therebetween and so that there will be no physical contact between the two. Furthermore, the dimensions are such that the electrode will rest on the bottom or closed end of the core with flange I2 above and not in contact with shoulder IS.

The solder or alloy which serves as the thermal coupling or heat transfer medium between the part to be cooled and the cooler or heat exchange means must have a melting or fusion point such that it is in the' liquid state well bef l'ow the operating themperature of the cooled part and the cooler and should not ailcy in the liquid state with either the cooled part or the cooler. Furthermore, such a heat transfer medium or solder must not cxidize seriously and must have sufficient thermal conductivity.

In connection with the illustrated electron discharge device, the accepted standard is core temperature of C. The which I prefer to use as the solder between an aluminum cooler and copper electrode is lead-bismuth eutectic (commercially availa" e Cerrobase; Cerro de Pasco Corp., New rk, N. Y.) which melts at 124 C. Such an a oy is solid at ordinary ambient temperatures countered while the device is not in opera handling of the electron disch? its cooler as a unit. During perature of the core C. while the solder alloy r .ch acts as the heat transfer medium betwe n the receptaclelike ccre and the electrode conpletely liquides well below this temperature in of its relatively low fusing temperature. Thus, in spite of the difference in the relative expansion and contraction of the aluminum core and copper electrode and in View of the liquid state of the heat transfer medium between the two, no mechanical stresses arise betr n the two above 124 C. and hence deformation of the copper electrode is avoided.

I have found that in practice as the solder alloy liquies or fuses a certain alor-.nt thereof is squirted upwards probably as a result of the central portion of the alloy melting thus setting up a slight pressure under the tcp crust of alloy which causes a small quantity to be squirted through the opening which appears in the top crust as the latter starts to melt. To avoid gradual depletion and loss of the alloy in the excess solder well, I provide a cover I8 in the form of a split ring as shown most clearly in Figure 3 which serves to clcse the excess solder well at the open end of core i 3. The well formed by portion I3 of core I3 holds an excess of solder over that required in the space between the core and the electrode to insure that the same is always completely filled and at the same time acts as a receiver for any solder alloy which may tend to overow from said space as a result of expansion of the liquid alloy.

As pointed out hereinabove the aluminum core is free to contract as it cools from operating temperature down to approx-mately 124 C. without imposing any stresses upon the copper elec trode. However, as the aluminum core continues to cool down to room temperature after solidification of the solder alloy it still tends to contract somewhat faster than the copper electrode. After only a single temperature cycle the resulting radial deformation of copper electrode is insignificant but since such deformations are cumulative with each subsequent temperature cycle, they may not be ignored if tube failure from this cause is to be avoided. For example, I have found in the case of tube type 9022 having a .600 inch aluminum core wall thickness, the radial deformation of the 4.590 inch diameter anode after 655 temperature cycles (equivalent to approximately 10,900 her life in normal service) vas .017 inch inw in diameter by test. I have shown that reducing the thickness of the aluminum core wall to substantially less than the'thickness of the cr to .250 inch will'eliminate even this reaz. sly deforma tion.

ge device and ation the temapproximately tains essentially approximately 52.5% bismuth,

32% lead and 15.5% tin, also by weight, and having a melting point of approximately 96 C.

In Figure 2 I have shown the heat exchange means as one adapted for use with a liquid coolant such as water. Electrode Il seats in receptacle-like portion 20 which may contain one or more conduits as for example tubing 2l wound in a double helix and connected to inlet 22 and outlet 23. An auxiliary inlet 24 and outlet 25 may be provided in convenient locations as indicated. A resistance element 2'6 may be conveniently utilized to heat and liquify the fusible alloy to facilitate removal of the device when not in operation from the cooler. ,In operation the alloy is in the liquid state and is in intimate contact with the coil and the electrode, affording an efficient thermal path therebetween, and resulting in considerably more uniform temperature distribution over the electrode than is normally the case when a coil is utilized which is merely wound around the electrode or connected thereto by solder which is solid during operation. Furthermore, in the case of the great power dissipation from an anode electrode where intense cooling is necessary it is no longer essential that the liquid wash over the electrode itself and subject the samef to a relatively large pressure as would be the case in a closed system. By using a fusible alloy the efficiency of the coil is greatly improved by the fact that it affords an interface area between the coolant and the electrode surface through the fusible alloy which is far greater than that which is the present limit of such systems where the water is in contact with the electrode surface.

If additional mechanical connection between the cooler and the electron discharge device is desired or if it is desired that they be mechanically connected during operation a few bolts may be readily used to clamp the device to the cooler as far example in the region of flange l2.

It is apparent from the foregoing that when a fusible alloy is utilized as the heat transfer medium between an aluminum cooler and a copper electrode deterioration of the thermal bond does not occur as would be the case when the usual solders are utilized where the difference in expansion tends on the one hand to cause separation of the solder from the aluminum and copper or both and on the other hand to cause permanent deformation of the copper electrode as a result of the compressive strains imposed during contraction of the aluminum. This as well as the improved liquid type cooler makes possible substantially improved high efliciency coolers with a substantial reduction in weight and cost. Thus, while I have set forth the foregoing embodiments of my invention and described it in detail with respect thereto, I do not wish to be limited to the exact construction set forth but desire to claim all modifications thereof that come within the scope of the appended claims.

What I claim is:

1. The combination with an electron discharge device having a part to be cooled, of a cooler having a coeicient of Vexpansion substantially different from that of the part to be cooled and including a receptacle-lilie portion, said part being seated in said portion, and a normally solid heat transfer medium forming a mechanical bond between said cooling unit and said device when solid and having a melting point below the normal operating temperature of said cooler for improved cooling of the part to be cooled.

2. The combination with an electron discharge device having a part to be cooled, of a cooling unit including a receptacle-like portion, said part being seated in said portion, a normally solid heat transfer medium connecting said part to said receptacle-like portion, and means in said receptacle-like portion for liquifying said heat transfer medium.

3. The combination with an electron discharge device having a metallic electrode including walls to be cooled, of a cooler having a coefficient of expansion substantially different from that of the electrode to be cooled and including a receptacle-like portion, the walls of said portion being substantially thinner than the walls of said electrode, said electrode being seated in said p0rtion, and a normally solid readily fusible heat transfer medium in contact with and between said electrode and said portion, said heat transfer medium having a fusion point below the normal operating temperature, and above the nonoperating temperature of said electrode and said portion, whereby said heat transfer medium fixes said cooler with respect to said electrode at said non-operating temperatures, and improves heat transfer from said walls of said electrode during said operating temperature.

4. In an electron discharge device, an electrode having life cycles of successive predetermined relatively low and predetermined relatively high temperatures, a heat exchanger surrounding and spaced from said electrode, and a heat transfer medium between said electrode and said heat exchanger, said heat transfer medium being fluid at said predetermined relatively high temperatures and solid at said predetermined lrelatively low temperatures, whereby said heat transfer medium mechanically fixes said heat exchanger with respect to said electrode during said relatively low temperatures and is characterized by improved heat transfer properties during said relatively high temperatures.

5. In combination, a device having -a part to be heated to a predetermined temperature and normally at a lower temperature than said predetermined temperature, a heat exchanger around and spaced from a portion of said part, and a heat transfer medium between said heat exchanger and said portion comprising a material having a melting point higher than said lower temperature for normally mechanically connecting said heat exchanger and said part to be heated, and lower than said predetermined temperature for fusing at said predetermined temperature to improve the thermal bond between said part and said heat exchanger, and for relieving strains due to different mechanical re- .aluminum cooler comprising a receptacle for receiving a portion of said electrode in spaced re- --lation with respect to the inner walls of said receptacle, and a heat transfer medium between said portion of said electrode and said inner walls, said medium comprising a material having a melting point below said predetermined temperature, whereby said material becomes uid at said predetermined temperature for improved contact with said electrode and said cooler and improved heat transfer from said electrode to said cooler, said material having a melting point above the normal temperature of said electrode for mechanically fixing said electrode with respect to said cooler during non-operating periods.

'7. A heat transfer medium for use between a part subjected to cycles of temperature variations from below 124 C. to above 124 C., and a heat exchanger, said medium comprising an alloy of bismuth and lead in eutectic forming pron portions, said alloy having a melting point of approximately 124 C., whereby said alloy is solid during relatively low temperature portions of said cycles for xing said part to said heat exchanger, and fluid during relatively high temperature portions of said cycles for improved heat transfer from said part to said heat exchanger.

8. A heat transfer medium for xing a part to be heated to a heat exchanger during dormant periods when said part is unheated and for improving heat transfer for said part to said heat exchanger when said part is heated, said medium comprising an alloy containing essentially of about 32 per cent bismuth, about 40 per cent lead, and about 8 per cent cadmium.

9. A heat transfer medium having a melting point below a predetermined temperature of operation of a part to be cooled and engaged by said medium comprising an alloy consisting essentially of about 52.5 per cent bismuth, about 8 32 per cent lead, and about 15.5 per cent tin, whereby said medium is fluid at said predetermined operating temperature for improved heat transfer from said part.

10. In combination, a device having a part to be heated to a predetermined temperature above 124 C. and normally at a lower temperature than said predetermined temperature. A heat exchanger around and spaced from a portion of said part, and a heat transfer medium between said heat exchanger and said portion, said heat transfer medium comprising an alloy of bismuth and lead in eutectic forming proportions, said alloy having a melting point of approximately 124 C., whereby said alloy fuses at said predetermined temperature to improve the thermal bond and relieve strains between said part and said heat exchanger, and is solid at said lower temperature for mechanically connecting said heat exchanger and said part to be heated.

MILTON B. LEMESHKA.

REFERENCES CTTED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,318,832 Birdsall Oct. 14, 1919 2,006,731 Chesnut July 2, 1935 2,186,563 Suydam Jan. 2, 1940 2,235,669 Conklin Mar. 18, 1941 2,324,533 Pearson July 20, 1943 2,440,245 Chevigny Apr. 27, 1948 FOREIGN PATENTS Number Country Date 349,719 Great Britain June 4, 1931 473,792 Great Britain Oct. 20, 1937 

